Inshot gas burner nozzles, such as used in furnaces and many appliances, generally comprise a venturi tube which diverges from its input end to an enlarged output end. In some constructions, a burner head insert made of sintered or powdered metal has outlet openings and is mounted in the outlet end of the tube. One example of such an insert is described in U.S. Pat. No. 5,186,620 to Hollingshead, which is incorporated herein by reference in its entirety. In operation, gas is injected into the inlet end of the nozzle, entraining air into the nozzle with it. This primary air/gas mix flows through the tube to the burner head or flame retention insert. The primary air/gas mix passes through the insert and burns as it exits the insert, thereby forming a cone of flame projecting from the outer face. Secondary air flows around the outside of the venturi tube and is entrained in the burning mixture around the outside of the insert in order to complete combustion.
Other inshot burner designs have incorporated an “insertless” configuration. These burners have an advantage of being less costly because they have less parts and lower material content. Examples of these types can be found in U.S. Pat. No. 5,445,519 to Sigler U.S. Pat. No. 5,833,449 to Knight. The disadvantage of these burners is that they have a very limited range of firing rates due to the relatively open discharge end, i.e., low port loading, with minimal flame retention features. A high firing rate will cause flame instability and flame lift off, which results in excessive noise and poor combustion, i.e., high carbon monoxide (CO). On the other hand, a low rate can cause flashback. Flashback is the burning of the gas within the burner nozzle itself, which can cause overheating and deterioration of the nozzle. There is therefore a need for an inshot gas burner nozzle that is inexpensive and easy to make while maintaining flame stability over a wide range of firing rates.